Why does Slush's scoring system punish heavily for adding another worker?

See below how my reward was wayy smaller from block #18526 after adding mighty 600 MH worker at 2013-06-12 07:07. My other 200 MH worker reward from block #18525 was 0.096% of next block's reward which had 200 MH worker running non-stop but 600 MH worker running only the last 12 minutes. Block 18527 had both my workers running.

Total network hash rate has jumped by nearly 10% to over 154Thash/s since last night. Diff estimated to increase to over 18 million by Sunday.

Now this is interesting ! Asicminer activating a lot of new block erupters out of the factory ?I wanted to invest in asic but as I said in another post I think that very soon 50gh/s will be the minimum to hash so I will refrain and sadly just watch very soon :/